The present invention relates to a semiconductor device and a manufacturing method therefor, and more particularly to a semiconductor device including different types of electronic elements such as a MOS transistor and a capacitor using different supply voltages formed on the same substrate, and a manufacturing method for such a semiconductor device.
In a recent LSI process, high integration of circuits and fine fabrication of electronic elements have been generally advanced. In association therewith, it has become necessary to simultaneously fabricate different types of electronic elements such as a MOS transistor, capacitor, and resistor using different supply voltages on the same substrate with an increase in number of steps minimized.
Further, finer fabrication is required to obtain a higher-performance MOS transistor, causing a reduction in supply voltage. This is accompanied by a thinner gate oxide film in this MOS transistor. Although a main MOS transistor is formed from a thin film, another MOS transistor to which a high voltage is applied for use in an I/O portion or an analog circuit is required to have a thicker oxide film according to a higher supply voltage.
In fabricating such different types of MOS transistors using different supply voltages, it is general to separately form gate oxide films having physically different film thicknesses. A fabrication method for such gate oxide films having physically different film thicknesses is shown in FIGS. 5A to 5D.
As shown in FIG. 5A, an oxide film 13 is formed by thermal oxidation on a semiconductor substrate 11 in which isolation dielectric films 12 are formed to isolate individual elements. As shown in FIG. 5B, only a region for fabricating a transistor employing a thick gate oxide film is masked with a photoresist R.
As shown in FIG. 5C, a portion of the oxide film 13 unmasked with the photoresist R is removed by wet etching using hydrofluoric acid or the like. After removing the photoresist R, thermal oxidation is performed again to form gate oxide films 13aand 13b having different film thicknesses as shown in FIG. 5D. The total film thickness of the gate oxide film 13a obtained by the first thermal oxidation and the second thermal oxidation is not equal to the sum of the film thickness of the oxide film 13 obtained by the first thermal oxidation and the film thickness of the oxide film obtained by the second thermal oxidation. Accordingly, optimum thermal oxidation is performed as the first thermal oxidation so that a desired film thickness is totally obtained.
By repeating the photoresist patterning step, the wet etching step, and the subsequent thermal oxidation step as mentioned above, different film thicknesses of oxide films can be further obtained.
Further, simultaneous fabrication of a MOS transistor and a capacitor is known conventionally. Such a fabrication method is shown in FIGS. 6A to 6D.
As shown in FIG. 6A, a suitable sacrificial oxide film 18 is formed by thermal oxidation on a semiconductor substrate 11 in which isolation dielectric film 12 are formed to isolate individual elements. As shown in FIG. 6B, a transistor forming region is masked with a photoresist R, and a high concentration of impurity ions is implanted through the sacrificial oxide film 18 into the semiconductor substrate 11 to thereby form an n.sup.+ semiconductor region or a p.sup.+ semiconductor region as one electrode 17 of the capacitor.
After removing the photoresist R and the sacrificial oxide film 18, a gate oxide film 13b of the MOS transistor and a capacitive oxide film 13c of the capacitor are simultaneously formed by thermal oxidation as shown in FIG. 6C. The capacitive oxide film 13c has a desired film thickness. This thermal oxidation is accelerated by crystal defects produced in the region where the impurity ions have been implanted with a high concentration (accelerated oxidation), so that the capacitive oxide film 13c becomes thicker than the gate oxide film 13b. Thereafter, polysilicon is deposited over the semiconductor substrate 11 and next patterned to thereby simultaneously form a gate electrode 14b of the MOS transistor and another electrode 14c of the capacitor as shown in FIG. 6D.
In this method, the MOS transistor and the capacitor can be formed on the same substrate simultaneously and efficiently without largely increasing the number of steps.
It is known that depletion under a gate electrode can be prevented by increasing an impurity concentration in the gate electrode (see Japanese Patent Laid-open No. 2000-277626).
However, the above-mentioned methods have the following problems.
In the case of separately forming oxide films having physically different film thicknesses, there arises a problem that the number of steps increases according to the different film thicknesses to be obtained. Further, in the wet etching step, the etching liquid such as hydrofluoric acid penetrates at the edges of the oxide film masked with the photoresist, causing damage to the oxide film to be left. As a result, any adverse effects are produced in the related transistor, thus possibly causing a reduction in reliability. Accordingly, heavy use of this method is undesirable.
In the case of simultaneously forming the transistor and the capacitor, a thicker oxide film can be obtained in the capacitor by performing ion implantation with a high dosage in forming the electrode in the semiconductor substrate of Si to thereby accelerate the subsequent thermal oxidation. However, such high-dosage ion implantation may cause a large damage to the semiconductor substrate, so that the quality of the oxide film is reduced to cause a reduction in reliability.
As mentioned above, in obtaining a high-performance MOS transistor as a main transistor for LSI, a physically thin gate oxide film is essential, and an electrical effective film thickness of this oxide film in operating the transistor must be maintained at a small value. Accordingly, it is necessary to meet the individual requirements of the thin-film transistor and the electronic elements requiring different oxide film thicknesses, such as a transistor and a capacitor using different supply voltages.